In telescopic observation of infrared phenomena it is necessary to be able to distinguish infrared radiation emanating from the object, e.g., a star to be observed from ambient radiation. This is ordinarily accomplished by tilting the secondary mirror in a cyclical fashion from the optical axis where it views the star or other object under observation to an angle off the optical axis where it views the sky minus the star. This is done twenty or more times a second and provides an a.c. signal made up during each half cycle of the background radiation plus radiation from the star and during each other half cycle of the background radiation alone. This permits the radiation as seen by an on axis detector representative of background radiation to be subtracted from the radiation as seen by the detector representative of background radiation plus radiation of the star leaving only radiation representative of the star alone. A serious disadvantage of this method is that the tilting or chopping by the secondary mirror introduces coma which degrades the resolving power of the telescope.
The present invention overcomes this problem and provides an on-axis coma free image of the object under observation at any tilt angle of the secondary mirror.